H. Aarts

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

Neutron measurements in near‐Earth orbit with COMPTEL

The fast neutron flux in near-Earth orbit has been measured with the COMPTEL instrument on the Compton Gamma Ray Observatory (CGRO). For this measurement one of COMPTELs seven liquid scintillator modules was used as an uncollimated neutron detector with threshold of 12.8 MeV. The measurements cover a range of 4.8 to 15.5 GV in vertical cutoff rigidity and 3° to 177° in spacecraft geocenter zenith angle. One of the measurements occurred near the minimum of the deepest Forbush decrease ever observed by ground-level neutron monitors. After correction for solar modulation, the total flux is well fitted by separable functions in rigidity and zenith angle. With the spacecraft pointed near the nadir the flux is consistent with balloon measurements of the atmospheric neutron albedo. The flux varies by about a factor of 4 between the extremes of rigidity and a factor of 2 between the extremes of zenith angle. The effect of the spacecraft mass in shielding the detector from the atmospheric neutron albedo is much more important than its role as a source of additional secondary neutrons. The neutron spectral hardness varies little with rigidity or zenith angle and lies in the range spanned by earlier atmospheric neutron albedo measurements.

COMPTEL as a Solar Gamma Ray and Neutron Detector

The imaging Compton telescope COMPTEL on the Gamma Ray Observatory has unusual spectroscopic capabilities for measuring solar γ-ray and neutron emissions. Flares can be observed above the 800 keV γ-ray threshold of the telescope. The telescope energy range extends to 30 MeV with high time resolution burst spectra available from 0.1 to 10 MeV. Strong Compton tail suppression facilitates improved spectral analysis of solar flare γ-ray emissions. In addition, the high signal-to-noise ratio for neutron detection and measurement provides new neutron spectroscopic capabilities. For example, a flare similar to that of 1982 June 3 will yield spectroscopic data on > 1500 individual neutrons, enough to construct an unambiguous spectrum in the energy range of 20 to 150 MeV. Details of the instrument response to solar γ-rays and neutrons are presented.

Reflection Grating Spectrometer on board XMM

The x-ray multi-mirror (XMM) mission is the second of four cornerstone projects of the ESA long-term program for space science, Horizon 2000. The payload comprises three co- aligned high-throughput, imaging telescopes with a FOV of 30 arcmin and spatial resolution less than 20 arcsec. Imaging CCD-detectors (EPIC) are placed in the focus of each telescope. Behind two of the three telescopes, about half the x-ray light is utilized by the reflection grating spectrometer (RGS). The x-ray instruments are co-aligned and measure simultaneously with an optical monitor (OM). The RGS instruments achieve high spectral resolution and high efficiency in the combined first and second order of diffraction in the wavelength range between 5 and 35 angstrom. The design incorporates an array of reflection gratings placed in the converging beam at the exit from the x-ray telescope. The grating stack diffracts the x-rays to an array of dedicated charge-coupled device (CCD) detectors offset from the telescope focal plane. The cooling of the CCDs is provided through a passive radiator. The design and performance of the instrument are described below.

The Spektr-RG X-ray calorimeter

Spatially resolved X-ray spectroscopy with high spectral resolution allows the study of astrophysical processes in extended sources with unprecedented sensitivity. This includes the measurement of abundances, temperatures, densities, ionisation stages as well as turbulence and velocity structures in these sources. An X-ray calorimeter is planned for the Russian mission Spektr Röntgen-Gamma (SRG), to be launched in 2011. During the first half year (pointed phase) it will study the dynamics and composition of of the hot gas in massive clusters of galaxies and in supernova remnants (SNR). During the survey phase it will produce the first all sky maps of line-rich spectra of the interstellar medium (ISM). Spectral analysis will be feasible for typically every 5° x 5° region on the sky. Considering the very short time-scale for the development of this instrument it consists of a combination of well developed systems. For the optics an extra eROSITA mirror, also part of the Spektr-RG payload, will be used. The detector will be based on spare parts of the detector flown on Suzaku combined with a rebuild of the electronics and the cooler will be based on the design for the Japanese mission NeXT. In this paper we will present the science and give an overview of the instrument.

Sciamachy instrument design

The primary scientific objective of SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) is the global measurement of trace gases in the troposphere and the stratosphere. SCIAMACHY comprises two high resolution optical spectrometers scanning the atmosphere simultaneously in nadir and in limb. It retrieves the concentrations of trace gases from observations of transmitted, back scattered and reflected light from the atmosphere in the wavelength range between 0.24 and 2.4 μm. Important ozone chemistry and greenhouse gases, including O3, NO2, N2O, CH4, CO and CO2 are measured. In this paper the instrument design is presented, carried out as part of a recently successfully completed phase A feasibility study for the first European Polar Platform (1997) and the ATMOS-Umwelt Forschungs Satellit (1995).

Comptel measurements of the omnidirectional high-energy neutron flux in near-earth orbit

On four occasions, twice in 1991 (near solar maximum) and twice in 1994 (near solar minimum), one COMPTEL D1 detector module was used as an omnidirectional detector to measure the high-energy (> 12.8 MeV) neutron flux near an altitude of 450 km. The D1 modules are cylindrical, with radius 13.8 cm and depth 8 cm, and are filled with liquid scintillator (NE213A). The combined flux measurements can be fit reasonably well by a product of the Mt. Washington neutron monitor rate, a linear function in the spacecraft geocenter zenith angle, and an exponential function of the vertical geomagnetic cutoff rigidity in which the coefficient of the rigidity is a linear function of the neutron monitor rate. When pointed at the nadir, the flux is consistent with that expected from the atmospheric neutron albedo alone. When pointed at the zenith the flux is reduced by a factor of about 0.54. Thus the production of secondary neutrons in the massive (16000 kg) Compton Gamma-Ray Observatory spacecraft is negligible. Rather, the mass of the spacecraft provides shielding from the earth albedo.

Back-illuminated CCDs developed for the reflection grating spectrometer on board XMM

Back-illuminated CCDs with high quantum efficiency in the soft x-ray range have been developed by EEV in collaboration with the Space Research Organization of the Netherlands (SRON) and the European Space Agency (ESA). These CCDs will be used as detector for the reflection grating spectrometer on board of the ESA x-ray multi-mirror mission XMM. To cover the full image of the reflection grating spectrometer an array of 9 CCDs along the Rowland circle with minimum dead space between the adjacent CCDs is needed. To obtain a high quantum efficiency over the full energy range (0.35 to 2.3 keV) the CCDs are illuminated from the backside. This requires a thin (approximately 50 nm) and homogeneous passivated layer at the backside, which is obtained by gas immersion laser doping. In addition a thin Al layer is deposited on the backside to reduce the sensitivity of the CCDs for visible/UV light. The technical aspects of the production of these CCDs as well as their calibration are discussed.

Neural Net Approaches for Event Location in the Detector Modules

From the description of the Compton telescope given previously (Schonfelder et al., this volume), one can see that the accuracy with which one determines the position of a cosmic gamma-ray source depends not only on the measurements of the energy deposited in the upper (D1) and lower (D2) detectors, but also on how accurately one estimates the (X, Y, Z) positions of each gamma-ray or neutron interaction (an event). If nothing were known about the position of each event except in which module it occured, it would increase the uncertainty in the position of a source by on the order of 10°. Within each COMPTEL module, one extracts position information from comparisons of relative intensities of signals in the photomultipier tubes. This technique was introduced in the 1950’s for medical imaging by Anger (1958), and later was adapted to astrophysical applications (Zych et al. 1983; Schonfelder et al. 1984; Stacy 1985).

X-Ray Characteristics Of CCD's for the XMM Reflection Grating Spectrometer (RGS)

The Reflection Grating Spectrometer (RGS) experiment on the X-ray Multi-Mirror (XMM) spacecraft will consist of two identical medium resolution ( ~100 - 560) reflection grating spectrometers operating in the 5-35 Å range.1 The dispersed X-ray beam will be detected by an array of 10 CCDs (nine for spectroscopy, one for wavelength and alignment calibration). This paper describes the requirements set upon CCD performance by this experiment as well as the first results obtained with a special test setup which has been designed as to allow optimum flexibility in changing all CCD parameters (clock sequences, slopes, bias voltages etc.) under software control.